Thin-film sensors fabrication

For thin-film metallization, a thin metallic film is first deposited onto the surface of the substrate. The deposition can be accomplished by thermal evaporation, electronic-beam- or plasma-assisted sputtering, or ion-beam coating techniques, all standard microelectronic processes. A silicon wafer is the most commonly used substrate for thin-film sensor fabrication. Other substrate materials such as glass, quartz, and alumina can also be used. The adhesion of the thin metallic film to the substrate can be enhanced by using a selected metallic film. For example, the formation of gold film on silicon can be enhanced by first depositing a thin layer of chromium onto the substrate. This procedure is also a common practice in microelectronic processing. However, as noted above, this thin chromium layer may unintentionally participate in the electrode reaction. 

In order to address the needs of field sensing of explosives, it is necessary to move away from traditional bulky electrodes and cells (commonly used in research laboratories). The exploitation of advanced microfabrication techniques allows the replacement of conventional ( beaker-type ) electrochemical cells and electrodes with easy-to-use sensor strips. Both thick-film (screen-printing) and thin-film (lithographic) fabrication processes have thus been used for high-volume production of highly reproducible, effective and inexpensive electrochemical sensor strips. Such strips rely on... 

Following these CNT electronic gas sensor studies, many other methods have been explored focusing on the reduction of fabrication cost. Snow et al. demonstrated that a low-density random network of SWCNTs can be fabricated into p-type thin-fllm transistors [Figure 14.7(c)] with a fleld-effect mobility of about 10 cm / Vs and an on-to-off ratio of about 10 [65]. They demonstrated that such thin-fllm transistors can detect dimethyl methylphosphonate (DMMP), a simulant for the nerve agent sarin, at sub-ppb levels [45]. SWCNT network transistors have also been transferred to polymer substrates to form flexible electronic gas sensors [66]. Other resistive sensors based on random SWCNT network [47] or MWCNT films [41] have also been reported. Besides the cost, CNT network and thin-film sensors increase the statistical reliability by averaging out the response at many adsorption sites. This is particularly important when gas concentration is extremely small. 

Patel, N. G., Makhija, K. K. and Panchal, C. J. (1994) Fabrication of carbon dioxide gas sensor and its alarm system using indium tin oxide (ITO) thin films. Sensors and Actuators B Chemical 21,193-7. 

In this chapter, we will describe and discuss the fabrication and characterization of metal oxide thick- and thin-film sensors (which are based on the conductivity modulation principle) and SAW devices (which are based on frequency modulation). The sensing properties of thick or thin semiconductor films with various components will be discussed, focusing on sensitivity and stability. In addition, SAW-type CWA sensors will be introduced for different pairs of inteidigitated transducer (IDT) fingers, various wave lengths, and different polymer coatings on the input and output IDTs of the device. 

Ceramic thin films, sensors, nanoscale materials, multi-functional ceramic composites, optical fibers, ceramic membranes and many other products can be manufactured by the sol-gel process [1-3]. The major applications of sol-gel processing are in ceramic industry for fabrication of oxide ceramics and glasses. Several studies have been reported on the preparation of supported catalysts and zeolite granular particles using the sol-gel technique [4-10]. Sol-gel derived inorganic thin films and membranes have recently attracted attentions from both academia and industry [11-13]. Only limited studies have been carried out on the sol-gel fabrication of adsorbents for industrial separation or purification purposes. 

Figure 2 shows a radio frequency magnetron sputtering apparatus for the fabrication of the thin-film sensor shown in Figure 1. The substrate (2) which is engine frictional part is fixed onto the substrate holder (1) in the top of the sputter chamber. The target (3) consists of sputter materials such as AI2O3, Si02 and sensor alloys. The film was deposited under the conditions listed in Table 1.

Thick/thin-film sensor can be fabricated oti prefabricated electrodes by dip/ spray-coating chemically synthesized polymer solution or by electropolymerization of monomer on the electrode from a solutimi in the presence of the dopant. The dip/ spray-coating process results in a 2-D stracture without control over the thickness, morphology, and porosity, and also does not provide the process flexibility to tune the material properties. On the other hand, electropolymerization processes such as cyclic voltammetry and chronoamperometry provide in situ fabrication on the electrode pads and also allow manipulating material properties by changing the process conditions. However, the film thickness in these methods cannot be controlled as there is no direct indication of the completion of the device fabrication process. Additionally, the contact between the gold electrode pad and the CP film and adhesion of the film to the substrate are issues for fabricarimi of CP-based... 

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